34 THE GEOLOGY OP MINNESOTA. 



[The Kekequabic granite. 



known to occur beneath the schists and also superimposed on them, but it is not 

 known, in this region, to occur as dikes in the schist, although it cuts some of the 

 associated clastic strata. The granite also rises in domes though considerably larger. 

 Geographically the porphyritic domes are so situated in relation to the granitic areas 

 that they can be considered peripheral phases of the granite, projecting further 

 amongst the elastics than the granite proper, but they were probably derived from 

 slightly different sources. 



Both the granite, which also passes into syenyte, and the porphyry, contain in 

 places numerous pebbly or bouldery inclusions, and in the field have been noted 

 several instances where megascopically the crystalline rock passes into the pebbly 

 green schist already described, and into a granite which is charged with pebbles. 

 These transitions seem to be in accord with microscopic transitions, as detailed below. 

 In the first place, it should be noted that there is a striking mineralogical affinity 

 between the schist conglomerate and the crystalline rock, in that the augite in 

 both is a3gyrine, and that the feldspars of the schist-conglomerate, having very 

 striking and unusual characters, are duplicated in the porphyry and in the granite, /. e., 

 the original feldspars are remarkably twinned and zoned. This statement as to the 

 augite is not demonstrated, but rests on the concurrent evidence of other microscopic 

 characters. It is evident that in such a schist it would be almost impossible to find 

 augite retaining its crystalline form, for it readily changes to hornblende, that being 

 indeed almost the first mineralogic change that takes place in a volcanic tuff of such 

 age. But the augitic cores remain in the schist, sometimes as augite (No. 1060), and, 

 on a still broader scale, the original forms of the augites are outlined in the resultant 

 hornblendes by differences of absorption and of colors between the nicols. Exactly 

 the same characters are seen in the augites of the porphyry where the preservation 

 is sufficiently perfect to disclose the aegyrine characters of the original augite. As 

 to the sameness of the feldspars, with their peculiarities, there is no question. 



These two important characters ally these rocks in some way in a genetic bond, 

 for the feldspars especially are wholly unlike any known elsewhere in the state. 

 Chemical analysis points to anorthoclase, but the zoned structure, when analyzed by 

 the microscope, indicates that the feldspars began as labradorite, passed to andesine, 

 and sometimes at least terminated as albite, there having been a continual increase 

 in the acidity of the material from which they were generated. 



The general aspect of the granite (seen in thin section) along the south side of 

 Kekequabic lake is suggestive, not of crystallization from a magma, but of simple 

 induration of coarse debris. The feldspar grains do not interlock, except as they 

 have been enlarged by a secondary growth, and in many sections examined they do 

 not even come into contact, but are separated, very generally, by a space which is 



